Experimental measurement of leakage flow in centrifugal blood pump

Abstract

In the design of a blood pump, leakage flow study is crucial as high leakage flow is

necessary for good washout within the pump, thereby preventing thrombus formation.

However, excessive leakage flow will lead to hemolysis due to high fluid shear stress.

Therefore, there is a need to determine the optimized clearance gap size.

Initially, investigation and adjustment of the experiment test rig was conducted to

improve the reliability of the instrument. Experiments were carried out at four gap sizes

and two rotational speeds to establish the pump characteristics, to measure the pressure

difference between the outer and inner radii of the impeller, and the leakage flow rate in

the clearance gap. To conduct leakage flow measurements and to ensure better accuracy

of the results, annular strips of shims were added onto the volute interior surface to

prevent the flow from flowing directly into the impeller eye.

The experimental results showed that the pump performance at a gap size of 0.17mm is

not significant due to high frictional torque generated at smallest clearance gap. A

proposed explanation for the high frictional torque was the high mean pressure measured

at 0.17mm gap due to high resistance of the flow. The leakage flow measurement results

showed that throttling, varying pump speeds and clearance gap size have a direct effect

on the measurement. At low operating flow rate, higher rotational speed and larger gap

size yield a higher leakage flow rate. A linear relationship between mean pressure and

leakage flow was found when the graph of mean pressure against leakage flow rate was

plotted. A comparison between experimental results with theoretical and CFX results

shows high accuracy of the measurements taken at 0.17mm gap. In general,

experimental results yield a lower measurement of leakage flow rate, and a proposed

interpretation of this result is the limited tapping points on the volute used to measure

the accurate amount of leakage flow in the gap region.